Reuse frames from normal speed animation to save time rendering slow motion
If you rename the already rendered files (using any batch renaming app) like make them all even or odd, then there is not much to do.

Change the speed of the animation using time remapping.



Read:
How can I modify the frame rate without changing overall animation speed?

Then disable the overwrite box in the output settings.



Existing frames will be skipped and only the missing numbers will be rendered.